Air bag safety systems have come into widespread use in automotive vehicles and light trucks and have been proposed for use in passenger trains and airplanes.
Such systems comprise an inflatable cushion, commonly referred to as an "air bag" which is stored folded in a storage receptacle and then very rapidly inflated, as with gas from a pyrotechnic gas generator, when a collision of the vehicle is detected by sensors. The air bag is thereby deployed in a position to absorb the impact of the driver or a passenger.
It is necessary that the folded air bag be stored in an enclosed secure environment within the passenger compartment, protected from tampering, and yet be allowed to properly deploy into the passenger compartment as the air bag is inflated.
It is critical that the air bag deploy within milliseconds of activation of the system in order to protect the occupant.
As noted, the air bag is enclosed within a storage receptacle, which is typically mounted behind an interior trim piece, such as a steering wheel cover in the case of the driver's side air bag, or a section of the instrument panel, in the case of the passenger's side air bag. It has been proposed to also provide side impact air bags in the vehicle doors.
One or more air bag deployment doors normally overlie the air bag receptacle and are forced open when the air bag is inflated to allow deployment of the air bag through the opening created by the door panel movement.
As described in U.S. Pat. No. 5,082,310 issued on Jan. 21, 1992 for an "Arrangement for Providing an Air Bag Deployment Opening", a seamless construction is advantageous in which the deployment door panels are not separately delineated within the expanse of the trim piece, but rather a smooth uninterrupted surface is provided extending over the deployment door substrate panels.
This construction necessitates severing portions of the covering of the trim piece in order to allow the door panels to hinge open.
Severing has been achieved by the pressure of the inflating air bag, or by various other methods which have been proposed, such as linear energy devices described in copending U.S. Ser. No. 08/279,225, filed Jul. 22, 1994. See also U.S. Ser. No. 08/027,114, filed Mar. 4, 1993, and U.S. Pat. Nos. 5,127,244 and 4,991,878 describing pyrotechnic elements used to cut the outer cover layer of the trim piece.
Cutter blades have also been proposed which are forced outwardly by the air bag inflation to assist in cutting the cover layer, but these outwardly swinging elements can present a potential hazard to a vehicle occupant seated in front of the deployment door.
Automotive interior trim covering materials such as vinyl plastic are relatively tough and difficult to sever, and also a predetermined severing pattern is necessary for proper door panel opening, such that heretofore preweakening grooves have been formed in the trim cover in a predetermined pattern to insure proper opening.
It has heretofore been proposed to provide an "invisible seam" installation in which the deployment door pattern is totally invisible to a person seated in the vehicle passenger compartment, and even faint outlines or "witness" lines are desirably avoided.
Scoring of the covering layer from the inside, if not done accurately, can over time become at least faintly visible from the exterior of the trim piece.
Fabrication of the automotive interior trim pieces with preweakening grooving particularly for invisible seam applications is thus a difficult manufacturing challenge.
First, the groove depth must be carefully controlled in order to achieve reliable rupture of the outer cover at exactly the right time during the air bag deployment event.
If the groove is too shallow, the thickness of the remaining material may be too great, presenting excessive resistance to severing, delaying air bag deployment. Conversely, if too little material remains, over time cracking may be result, or at least allow the appearance of externally visible "witness" lines.
The preweakening effect may also be less effective if the grooves are molded-in during the process since it has been found that cutting into plastic material such as vinyl has a better preweakening effect compared to molding-in the groove during the initial manufacture of the item.
The high pressures used in injection molding can cause a "crazing" effect at the thinned bridging material extending over the gap defined by the groove. This crazed zone is rendered more visible as the part is removed from the mold, particularly if the part is not completely cooled when it is being removed.
The net effect is that the molded groove becomes visible on the exterior side.
It is difficult to accurately and reliably control the depth of mechanical cutting of component materials such as sheet vinyl, since the material is variably compressed by the pressure of a cutting instrument.
U.S. Pat. No. 5,082,310, referenced above, describes a partial cutting procedure which is intended to enable accurate control over the depth of the cut into a sheet of pliant plastic material such as a vinyl skin. However, a purely mechanical cutting operation still has other inherent accuracy limitations and is slow to execute.
Also, some cover materials have irregular inside surfaces, i.e., dry powder slush processes create such irregularities. If the groove depth were constant, this results in an irregular thickness of the remaining material. This leads to erratic performance as the resistance to opening pressure will vary greatly.
The groove width is also important, in that if a too narrow groove is cut into many plastics, a "self healing" may occur, particularly at elevated temperatures in which the groove sides will re-adhere to each other, causing the preweakening effect to be erratic or neutralized.
The required groove width also varies with the notch sensitivity of the material being preweakened.
A further difficulty is encountered in assembling the preweakened component to the interior trim structure so that the lines of preweakening are properly registered with the other components. For example, the vinyl skin in a skin and foam instrument panel must be accurately positioned on the instrument panel substrate and the deployment door substrate panels so that the preweakening lines are stressed as the door edges hinge out under pressure from the air bag.
This alignment requirement creates manufacturing difficulties and increased costs particularly since a variety of forms of instrument panel structures are employed, i.e., skin and foam, vinyl clad, hard plastic with a finished surface, etc., since a variety of forming techniques are employed, i.e., vacuum formed calendared plastic sheet, day powder slush molded, injection molded, etc. A leather covering later sometimes may be used in lieu of a vinyl plastic covering layer.
Accordingly, it is an object of the present invention to provide a process for preweakening trim components overlying an air bag installation by groove scoring which is highly accurate in production implementation, and which may be efficiently integrated into the trim piece manufacture to lower costs and improve results.